1. Field of the Invention
This invention is a method for bonding a skin member such as metal, plastic, or reinforced plastic, to a honeycomb core material such as a joined set of hexagonally shaped hollow tubular cells. The method has applicability in the construction industry and in the aircraft and satellite manufacturing industries.
2. Description of the Prior Art
A simple prior art approach for bonding metal or another skin member to a honeycomb core material is shown in FIG. 3, wherein skin 16 is attached to the vertical elongated walls 12 of the honeycomb cells. Adhesive layer 14 is applied to skin 16, and this combination is simply placed on top of cell walls 12 with or without pressure and heat. A bond is thus formed between skin 16 and cell walls 12 by using adhesive 14. (The drawings are not drawn to scale in the sense that cell walls 12 are typically thinner than shown). The problem with this method is that it uses a lot of unnecessary adhesive, the weight of which is undesirable in the aircraft and satellite manufacturing industries. The present invention uses half the adhesive (by weight) as this prior art method for the same thickness. Also, this prior art method cannot be used with thin acoustic skins as the precured member 16, since the adhesive 14 would coat and seal up the acoustic perforations in the skin and severely degrade the acoustic properties of the sandwich structure.
Another approach used in the prior art is known as roller coating, exemplified in FIG. 4, wherein a glue or adhesive in liquid or paste form is rolled onto the ends of honeycomb structure 12. The glue is labeled 18. The problem with this method is that it is difficult to control the amount of glue, which is dependent on the skill of the operator. Further, it is impossible to use this method with the newer structural film adhesive materials which tend to have better structural and environmental properties that are desired for satellite and aircraft applications. This method is exemplified by U.S. Pat. No. 2,609,068. This method is also illustrated in U.S. Pat. No. 3,817,810 and results in the adhesive being coated onto the edges of the honeycomb cells only. The method is expensive and can be used only with flat core. It is totally operator dependent and not desirable for high rate production, particularly where reliability and reproducibility of the adhesive coated core is required. On the other hand, the present invention does not require the use of any specialized equipment, is reproducible, and can be used with curved faying honeycomb surfaces.
U.S. Pat. No. 2,477,852 is an old patent covering a glass-fiber honeycomb and the bonding of a noncured material such as a prepreg to the core. This patent does not give details concerning the adhesive but the implication is that either no adhesive is used and the prepreg resin acts as an adhesive, or the roller coating method is used. Column 5 lines 27-30 state, "Additional adhesive may be applied to the attaching surfaces of the core, that is, the end faces of the assembled tubes, to assure thorough sealing at the intersections".
A similar patent is that of U.S. Pat. No. 3,700,522 where again the adhesive goes only on the ends of the cell walls of paper honeycombs. An uncured adhesive is used and is probably applied by roller coating.
A method similar to the roller coating method is known as curtain coating, which utilizes an expensive machine. The curtain coating technique for adhesive application is again limited to liquid adhesive materials. It is difficult to control the curtain coating of honeycombs and only flat surfaces can be coated.
An additional prior art method is the reticulating film adhesive method, illustrated in FIG. 5 of the drawings. In that method an adhesive film with a back-up plastic release film or paper layer is applied to the top surface of the honeycomb, adhering to the edges of the honeycomb cells. Then the back-up release layer is removed and the adhesive, which has formed membranes across the cells of the honeycomb, are burst like bubbles with the simultaneous application of heat and air pressure from above. When the film adhesive membrane "pops" all the adhesive remains and draws back to the core walls as shown as items 20 on FIG. 5. The problems with this method are that too much adhesive remains, and the method can be used only with a special class of reticulating adhesives. It also requires special equipment for rapid heating and applying air pressure to the adhesive covered core.
U.S. Pat. No. 3,674,595 shows a pressure sensitive adhesive applied to a porous substance. The present invention differs from this patent in that a porous substance is not a faying material and a release paper is not used.
U.S. Pat. No. 3,921,368 described freezing a layer of water in a process for gluing together two corrugated paper surfaces.
In U.S. Pat. No. 3,089,800, adhesive is applied to just the raised surfaces of the material to be bonded, unlike the present invention.
None of the above references shows a simple, inexpensive, and reproducible method for bonding a skin member to a honeycomb structure, wherein adhesive is not applied along the surface formed by the joining together of the two members.